In a typical radio communications network, wireless terminals, also known as mobile stations, wireless devices and/or user equipments, UEs, communicate via a Radio Access Network, RAN, to one or more core networks. The RAN covers a geographical area which is divided into cell areas, with each cell is served by a base station, e.g. a radio base station, RBS, or network node, which in some networks may also be called, for example, “NodeB”, “eNodeB” or “eNB”.
A Universal Mobile Telecommunications System, UMTS, is a third generation mobile communication system, which evolved from the second generation, 2G, Global System for Mobile Communications, GSM. The UMTS terrestrial radio access network, UTRAN, is essentially a RAN using wideband code division multiple access, WCDMA, and/or High Speed Packet Access, HSPA, for user equipments. In a forum known as the Third Generation Partnership Project, 3GPP, telecommunications suppliers propose and agree upon standards for third generation networks and UTRAN specifically, and investigate enhanced data rate and radio capacity. In some versions of the RAN as e.g. in UMTS, several base stations may be connected, e.g., by landlines or microwave, to a controller node, such as a radio network controller, RNC, or a base station controller, BSC, which supervises and coordinates various activities of the plural base stations connected thereto. The RNCs are typically connected to one or more core networks.
Specifications for the Evolved Packet System, EPS, have been completed within the 3rd Generation Partnership Project, 3GPP, and this work continues in the coming 3GPP releases. The EPS comprises the Evolved Universal Terrestrial Radio Access Network, E-UTRAN, also known as the Long Term Evolution, LTE, radio access, and the Evolved Packet Core, EPC, also known as System Architecture Evolution, SAE, core network. E-UTRAN/LTE is a variant of a 3GPP radio access technology wherein the radio base station nodes are directly connected to the EPC core network rather than to RNCs. In general, in E-UTRAN/LTE, the functions of a RNC are distributed between the radio base stations nodes, e.g. eNBs in LTE, and the core network. As such, the Radio Access Network, RAN, of an EPS has an essentially flat rather than hierarchical architecture comprising radio base station nodes without reporting to RNCs.
Regardless of the wireless communications technology used in the radio communications network, a cell is a geographical area where radio coverage is provided by the radio base station at a base station site or an antenna site in case the antenna and the radio base station are not collocated. Each cell is identified by an identity within the local radio area, which is broadcast in the cell. Another identity identifying the cell uniquely in the whole mobile network is also broadcasted in the cell. One base station may have one or more cells. The base stations communicate over the air interface operating on radio frequencies with the user equipments within range of the base stations.
Frame Structure and Reference Symbols, RS, in LTE
Even though not limited to any wireless communications technology, reference and explanations may be provided herein with respect a LTE network. Thus, a brief overview of the LTE frame structure and Reference Symbols, RS, is provided below.
LTE is a Frequency Division Multiplexing, FDM, technology, wherein Orthogonal Frequency Division Multiplexing, OFDM, is used in e.g. a downlink, DL, transmission from a eNB to a UE. The basic LTE downlink physical resource may thus be seen as a time-frequency grid as illustrated in FIG. 1, where each Resource Element, RE, corresponds to one OFDM subcarrier during one OFDM symbol interval. A symbol interval comprises a cyclic prefix (cp), which cp is a prefixing of a symbol with a repetition of the end of the symbol to act as a guard band between symbols and/or facilitate frequency domain processing. Frequencies f or subcarriers having a subcarrier spacing Δf are defined along an z-axis and symbols are defined along an x-axis.
In the time domain, LTE DL transmissions are organized into radio frames of 10 ms, each radio frame comprising ten equally-sized subframes, #0-#9, each with a Tsubframe=1 ms of length in time as shown in FIG. 2. Furthermore, the resource allocation in LTE is typically described in terms of Resource Blocks, where a RB corresponds to one slot of 0.5 ms in the time domain (7 OFDM symbols) and 12 subcarriers in the frequency domain. RBs are numbered in the frequency domain, starting with resource block 0 from one end of the system bandwidth. Thus, an RB consists of 84 REs.
DL and UL transmissions are dynamically scheduled, i.e. in each subframe the eNB transmits control information about to or from which UEs data is transmitted and upon which RBs the data is transmitted. The control information for a given UE is transmitted using one or multiple Physical Downlink Control Channels (PDCCH). Control information of a PDCCH is transmitted in the control region comprising the first n=1, 2, 3 or 4 OFDM symbols in each subframe where n is the Control Format Indicator (CFI). Typically the control region may comprise many PDCCH carrying control information to multiple UEs simultaneously. A downlink system with 3 OFDM symbols allocated for control signaling, for example the PDCCH, is illustrated in FIG. 3 and denoted as control region. The REs used for control signaling are indicated with wave-formed lines and REs used for reference symbols are indicated with diagonal lines. Frequencies f or subcarriers are defined along an z-axis and symbols are defined along an x-axis.
The signal transmitted by the eNB in a DL subframe may be transmitted from multiple antennas, and the signal may be received at a UE that has multiple antennas. The radio channel distorts the transmitted signals from the multiple antenna ports. In order to demodulate any transmissions on the DL, a UE relies on the RS that are transmitted on the DL. In addition, RS may be used to measure the channel between the transmitter and the receiver antenna. Therefore, Antenna Ports, AP, is introduced in the LTE specifications. Each RS is associated with an AP. When the UE is measuring the channel using the RS, it may be referred to as the UE is measuring the channel from the stated AP to its receiver antenna. It shall be noted that it is up to transmitter implementation how to transmit the RS in case there are multiple physical antennas at the transmitter side used to transmit the RS for a single AP. The mapping of a RS to multiple physical antennas is called antenna virtualization and this operation is transparent to the UE, since the UE may only measure the channel on the given RS, i.e. the AP.
The RS and their position in the OFDM time-frequency grid are known to the UE. Hence, this may be used to synchronize to the DL signal and determine channel estimates by measuring the effect of the radio channel on these RS. In Release 11 LTE network, and in prior releases, there are multiple types of RS. The Common Reference Symbols, CRS, which corresponds to AP 0-3, are used for channel estimation during demodulation of control and data messages in addition to synchronization. The CRS are present in every subframe. The Channel State Information Reference Symbols, CSI-RS, which correspond to AP 15-22, are also used for channel state feedback related to the use of transmission modes that enable UE-specific antenna precoding. These transmission modes use the UE-specific Demodulation Reference Symbols, DM-RS, which correspond to AP 7-14, at the time of transmission with the precoding at the eNB performed based on the feedback received from and measured by the UE on the CSI-RS.
Furthermore, a primary synchronization signal, PSS, and a secondary synchronization signal, SSS, are used for cell search and coarse time and frequency synchronization. These signals are strictly not reference signals but synchronization signals and hence do not correspond to any numbered antenna port in the LTE specifications. FIG. 4 shows all of the above reference signals, i.e. CRS, CSI-RS, DM-RS, PSS, SSS, over two subframes of duration 1 ms each.
FIG. 5 shows a RE grid over an RB pair depicting the potential positions for CRS, CSI-RS, DM-RS. Here, the potential positions for CSI-RS are marked with a number corresponding to the CSI-RS AP.
The CSI-RS are modulated using a sequence that depends on a configurable, or virtual, cell ID that can be different from the cell ID being used in the cell. The CSI-RS also utilizes an orthogonal cover code of length two to overlay two APs on two consecutive REs. Many different CSI-RS patterns are available. For example, in case of 2 CSI-RS APs, there are 20 different patterns within a subframe. The corresponding number of patterns is 10 and 5 for 4 and 8 CSI-RS APs, respectively. For TDD, some additional CSI-RS patterns are available.
The PSS and SSS define the cell ID of the cell. The SSS may take 168 different values representing different cell ID groups. The PSS may take three different values that determine the cell ID within a group. Thus, there are a total of 504 cell IDs. The PSS are Zadoff-Chu sequences of length 63 which along with 5 zeros appended on each edge occupy the 73 subcarriers in the central 6 RBs. The SSS are two m-sequences of length 31 that occupy alternate REs and are appended with 5 zeros on each edge and located in the central 6 RBs as is the case for the PSS. The PSS and SSS sequences occur in subframes #0 and #5. The PSS is the same in both subframe #0 and #5 while the SSS sequences differs between the subframes. The sequence transmitted in subframe #0 is referred to as SSS1 while the sequence transmitted in subframe #5 is referred to as SSS2. The sequence, SSS2 swaps the two length-31 m-sequences transmitted as part of the sequence SSS1 in subframe #0.
Two APs, even belonging to different RS types, such as, e.g. CSI-RS and DMRS, may be identified as quasi-co-located, QCL, if some of the large scale channel properties, such as, delay spread, Doppler spread, Doppler shift, average gain and average delay corresponding to one AP may be inferred from the other AP. Which AP that are QCL and under what circumstances are given in 3GPP TS 36.213.
Discovery Signals
Dense deployments of small cells are attractive to increase system capacity in the radio communications. However, dense deployments typically have fewer UEs connected to each cell and lower resource utilization, with higher rates provided when the cells are used. RS structures that are developed for regular deployments with existing systems, such as, e.g. a 3GPP LTE network, may have too high a density so that there is a lot of unnecessary interference created, within or between cells, when deployments become dense. For example, RS may be transmitted even when there is no data being sent to UEs.
In order to tackle this problem of unnecessary interference, solutions to turn small cells off when they are not being used are being considered. However, to ensure that cells can be ready to deliver data to and receive data from UEs with minimal delay, it is necessary for UEs to make some essential measurements on cells even when they are off. In order to facilitate this, a set of RS that are sent with much lower density in time have been discussed. Such RS signals are commonly referred to as discovery signals and procedures associated with them as discovery procedures.
In a Release 12 LTE network, for a small cell on/off where the eNB can be off for long periods of time in order to assist the UE with the measurements, a discovery signal might be needed. The discovery signal needs to support the properties required for enabling RRM measurements, RLM related procedures and coarse time/frequency synchronization. In order to make the UE measurements possible, the eNB has to wake up periodically, e.g. once every 80 ms, or 160 ms, etc., and send the discovery signal so that it can be used by the UE for mobility related operations, such as, e.g. cell identification, RLM and measurement. Within one cell, there may be multiple TPs from which the DL signal may be transmitted. One example of this, it a Distributed Antenna System, DAS, wherein multiple radio remote heads that are physically dislocated within the cell transmit signals that all belong to the same cell, i.e. same Cell-ID. The term TP may also refer to a sector of a site where the different sectors of the same site then constitute different TPs. The discovery signal should also be capable of identifying individual TPs and enabling RRM measurements for them.
In addition to the ability to turn cells on and off, it is also beneficial for discovery signals to be able to allow UEs to make received power and quality measurements, such as, e.g. Reference Signal Received Power, RSRP, and Reference Signal Received Quality, RSRQ, measurements in an LTE network, for individual Transmission Points, TPs, that may be a part of a cell where the TPs may be geographically separated within the cell. This may facilitate the turning off and on of individual TPs, as well as, allow the cell to determine how to configure measurements of TPs of UEs for the purpose of obtaining more detailed Channel State Information, CSI, estimations.
Signals that may be deployed independently over multiple TPs exist in radio communication networks today. For example, in a LTE network, these are the Channel State Information Reference Signals, CSI-RS, as described above. However, simple RSRP and RSRQ measurements are not currently defined for them.
Definition of such measurements based on the CSI-RS and its use as a discovery signal is currently being discussed. However, as can be seen from the above, the CSI-RS have a high degree of configurability and are designed to be used for CSI measurements by the UE. Thus, due to the high degree of configurability, the UE needs to be provided assistance information by the network about the precise CSI-RS configuration that the UE should use. The provision of such information increases network complexity, since such information is typically not required currently for RSRP and RSRQ measurements. In addition, due to their sparseness, their measurement performance is not as robust as the performance based on the currently used Cell-Specific Reference Signals, CRS.
From the discussion above, it may be concluded that there is a need to provide a discovery signal which do not suffer drawbacks in measurement robustness or in the necessity to provide extensive assistance information from the eNB to the UE.